Abstract: Citrus tristeza virus (CTV) represents a serious threat to the Moroccan citrus industry, since the CTV-sensitive sour orange remains the major rootstock. CTV has been recorded several times, as isolated foci, over the years in the country; fortunately, whenever CTV was found, immediate eradication actions were taken. Recently, CTV was reported on a large extent in Northern Morocco in the Loukkos region and characterization studies highlighted the presence of high genetic diversity among CTV isolates in this region. Transmission experiments by Aphis gossypii (Glover) and Aphis spiraecola (Patch) on two Moroccan CTV isolates (mild and severe), were carried out in laboratory. The obtained results showed that A. gossypii was able to transmit efficiently the L-Clem2-Mor mild isolate (40%), while less was the transmission rate (13.33%) for the L-Clem1-Mor severe isolate. The mild isolate was transmitted at a rate of 6.67% by A. spiraecola, which was not able to transmit the severe one. The transmission trials showed that the mild CTV isolate was more transmissible than the severe one; this may explain the prevalence of the mild isolate in the region of Loukkos. Meanwhile, field CTV isolates were compared to their correspondent aphid derived sub-isolates using SSCP and sequencing analysis of the p18, p23 and p25 genes. SSCP analysis showed that the CTV isolates had the same migration patterns as their correspondent aphid derived sub-isolates, whereas, sequencing analysis revealed very limited mutations.